1. Field of the Invention
The present invention relates to an optical transmitter for generating an optical output signal based on an electrical signal.
2. Related Background of the Invention
IEEE 802.3ba and IEEE 802.3bg are formulated as standards of optical transmission networks in order to resolve a bottleneck of a transmission capacity of communication between servers in a data center and the like. In response to these standards, transmission rate of an optical transmission system or an optical transceiver mounted to the optical transmission system is required to be extended to 40 Gbps or 100 Gbps. In order to generate an optical output signal having high quality, the transmission rate of which is 40 Gbps, an optical transmitter (including a transmission part of an optical transceiver) utilizes an Electro-Absorption (EA) modulator as an optical modulator, and utilizes a modulator driving circuit to drive the EA modulator. Further, a control circuit is embedded in the optical transmitter. The control circuit receives setting information from a host system to control each of parts inside the optical transmitter, and concurrently monitors each of the parts, to notify the host system of various information such as an alarm and so on.
As a modulator driving circuit, for example, a circuit having a cross point adjustment function for adjusting a cross point of a waveform of an optical output signal is disclosed in Patent Literature 1. Further, a circuit for adjusting a cross point of a driving signal output from a semiconductor laser driving circuit is disclosed in Patent Literature 2.
Further, the optical transmitter properly controls the optical output signal according to the monitoring the status of the parts. For example, a wavelength division multiplex transmission system disclosed in Patent Literature 5 performs a control to maintain an optical output signal at a proper power level even when an input signal of a transmission system, having a certain wavelength, is shut off or in a non-synchronous state. Further, an optical transmitter disclosed in Patent Literature 6 controls an optical output signal by coping with a root cause of an anomaly in an automatic power control.
Patent Literature 1: Japanese Patent Laid-Open Publication No. 2000-59317.
Patent Literature 2: Japanese Patent Laid-Open Publication No. 2004-312396.
Patent Literature 3: Japanese Patent Laid-Open Publication No. 2001-257645.
Patent Literature 4: Japanese Patent Laid-Open Publication No. 2003-348021.
Patent Literature 5: Japanese Patent Laid-Open Publication No. 2002-44035.
Patent Literature 6: Japanese Patent Laid-Open Publication No. 2007-325189.
The above-mentioned optical transmitter corresponding to the related prior art performs a control to shut off a multiplexed electrical signal in order to prevent abnormal random noise from being output to the optical transmission network when an electrical signal or a reference clock given from the host system loses a predetermined frequency or a sufficient amplitude. After the control to shut off an optical output signal is performed, when the electrical signal or the reference clock is recovered, a control to resume the output of the optical output signal is generally performed.
Further, a method of a feedback control based on an optical output power as disclosed in Patent Literature 3, or a method of preventing excessive light emission by attaching a device for limiting optical output power to the outside regarding an optical transmitter using a direct modulation laser diode as disclosed in Patent Literature 4 has been known.
Therefore, an aspect of the present invention is to provide an optical transmitter which can continue normally operation when a clock signal or an electrical signal given from the outside by a host system or the like goes into an abnormal state and comes back from the abnormal state.